1. Field of the Invention
The present invention relates to improvements in a shift control system in an automatic transmission for an automotive vehicle.
2. Description of the Prior Art
It is well known that an automatic transmission for an automotive vehicle is shiftable in speed ratio by selecting a predetermined speed by selective hydraulic actuation of friction elements such as a clutch and a brake to change the friction elements to be operated. Accordingly, some types of shifting necessitate redeployment of the friction elements so that one friction element or high clutch is engaged while the other friction element or band brake is released as is seen during 2.fwdarw.3 shift of the automatic transmission described in a Maintenance Manual for NISSAN RE4F02A type Automatic Trans-axle, published by NISSAN MOTOR CO. LTD., in March, 1988. In such a shifting, a shift feeling is largely effected by the relationship of the raising of the engagement pressure and the lowering of the disengagement pressure.
However, since in the above-mentioned 2.fwdarw.3 shift the lowering and raising controls of the hydraulic pressure for the redeployment of the friction elements is not individually controlled, the shift feeling by the conventional shifting has a limit in its improvement. Furthermore, it is necessary to control the timing of the redeployment of both friction elements to be operated in the relation therebetween in order to obtain a preferred shift feeling. For example, when the raising of engagement pressure P.sub.ON is compared with the decrease of disengagement pressure as shown in FIG. 9, racing of engine speed due to accelerating downshift, or falling of the engine speed due to upshift (though not shown in FIG. 9) is caused. These changes of the engine speed generate a torque peak as shown by a torque curve T.sub.OS in FIG. 9, and this causes a shift shock which degrades shift feeling. On the other hand, although a one-way clutch for the shifting is often installed in such an automatic transmission and used for carrying out a smooth shifting, it is necessary that such a one-way clutch is disposed along an inner side of a transmission case. This increases a diameter of the transmission in size.